Mixed layer silicates typified by clay minerals and mica minerals are classified in detail on the basis of the constituent elements and the layer charges thereof. The basic structure of the layers of layered silicate is primarily composed of a tetrahedral sheet and an octahedral sheet. In the tetrahedral sheet, tetrahedrons in which four O2− coordinate with a metal, e.g., silicon or aluminum, are bonded into a hexagonal network shape so as to form a sheet. In the octahedral sheet, octahedrons in which six OH− or O2− coordinate with a trivalent, divalent, or monovalent metal, e.g., aluminum, magnesium, or lithium, are joined sharing edges. This tetragonal sheet and the octagonal sheet are joined sharing apex oxygen. A layer in which one tetrahedral sheet is bonded to one octahedral sheet is referred to as a 1:1 layer, and a layer in which tetrahedral sheets are bonded to both sides of one octahedral sheet is referred to as a 2:1 layer.
Smectite having a 2:1 layer falls roughly into di-octahedral smectite and tri-octahedral smectite. In many cases, trivalent aluminum is present in the octahedral sheet in the former, and divalent magnesium is present in the latter. Regarding each of them, if a shortage of positive charge occurs in the octahedral sheet or the tetrahedral sheet because of substitution with metals having different valences, the charge of the entire layer becomes negative. An absolute value of charge (negative) of a layer with reference to an ideal chemical composition is referred to as a layer charge. In order to keep a charge balance between layer charges, exchangeable cations are included between the layers. The names of minerals correspond thereto, and montmorillonite, beidellite, and the like are known as the di-octahedral smectite.
Saponite, hectorite, stevensite, and the like are known as the tri-octahedral smectite. Smectite is a fine particle clay mineral, and the layer charge per unit cell is within the range of 0.2 to 0.6. Specific properties, e.g., the ion exchange ability manifested from the layer charge, the swelling property, the dispersibility, and the intercalation function are exhibited. In contrast to other clay minerals, e.g., kaolinite to be used as a pottery pigment, smectite is used for special, industrial purposes, e.g., drilling mud, casting sand, organic smectite, and polymer-clay nanocomposite.
Likewise, examples of 2:1 layers having a layer charge of 0.6 to 1.0 include vermiculites; mica clay minerals typified by illite, sericite, glauconite, celadonite, and the like; and mica minerals typified by phlogopite, biotite, muscovite, palagonite, and the like. The crystallinity in directions of the a axis and the b axis is high and sheet area of each sheet is large as compared with a smectite crystal. However, nonexchangeable potassium ions are often included between the layers, and swelling property with water is not exhibited in contrast to smectite.
On the other hand, examples of 2:1 type layered silicates which do not manifest a layer charge structurally include talc in which magnesium has primarily substituted in an octahedral sheet and pyrophillite in which aluminum has substituted. They do not exhibit cation exchangeability. They do not swell nor disperse in water in contrast to smectite. Talc is a tabular crystal, and is industrially used as fillers for thermoplastic resins and the like for various purposes.
Among natural layered silicates, some layered silicates have structures in which unit structure layers of two or three types of clay minerals are laminated, and they are referred to as mixed layer minerals. Most of mixed layer minerals are generally mixed layer minerals composed of non-swelling layer/swelling layer. Regarding the naturally found mixed layer minerals having regularly laminated structure, for example, muscovite/smectite (montmorillonite), chlorite/smectite, mica/smectite, and talc/smectite (saponite) have been reported (Non-Patent Document 1).
Most of mixed layer minerals are specific materials having structures in which non-swelling layers and swelling layers are mixed and laminated and, therefore, have the properties of both layers in combination. It is expected that new high-performance materials are created by controlling this mixed layer structure. However, only a small amount of mixed layer minerals having uniform structures and good quality are naturally found. Therefore, development of uses for industrial materials is hardly considered.
On the other hand, some attempts to synthesize mixed layer silicates targeting for industrial use have been reported. For example, a synthetic mixed layer silicate including serpentine and smectite as constituent layers and a process for producing the same in which cations and, if necessary, fluorine ions are added to a composite hydrous oxide including silicon, magnesium, and aluminum as components so as to prepare a slurry and a hydrothermal reaction is conducted (Patent Documents 1 and 2), a mixed layer silicate including a fluorine containing mica-like structure portion and a talc-like structure portion and produced by heating a mixed fine powder containing an alkali metal selected from sodium and/or lithium, magnesium, silicon, oxygen, hydrogen, and fluorine as primary constituent elements at 700° C. to 1,200° C. and a process for producing the same (Patent Document 3), and the like have been reported.
The inventors filed an application for patent of the invention related to a nanocomposite having a high aspect ratio, which had not been reported previously, based on peeling of non-swelling mica having large crystal grains, which was a system not attempted previously (Patent Document 4). This is an attempt to research and develop a nanocomposite having a specific morphology by designing a layered silicate which becomes a dispersion phase. The mixed layer silicate of that invention is a characteristic inorganic layered substance in which swelling layers and non-swelling layers are laminated regularly, and a new nanocomposite can be produced by peeling and dispersion thereof.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 8-59226    Patent Document 2: Japanese Unexamined Patent Application Publication No. 9-268010    Patent Document 3: Japanese Unexamined Patent Application Publication No. 9-235116    Patent Document 4: Japanese Patent Application No. 2004-Non-Patent Document 1: Crystal structures of clay minerals and their X-ray identification, Edited by G. W. Brindley and G. Brown, Mineralogical Society, London, pp. 249-303, 1980.